Sintering Effects on Additive Manufactured Ni-Mn-Ga Shape Memory Alloys: A Microstructure and Thermal Analysis

This work investigates the effects of time dependency for isothermal sintering on additive manufactured Ni-Mn-Ga magnetic shape memory alloys. Binder jetting additive manufacturing was used to produce Ni-Mn-Ga parts from pre-alloyed powders. Additive manufacturing via the binder jetting technique produces parts with intrinsic porosities, based on the morphology of the source material. The Ni-Mn-Ga parts printed in this study using the binder jetting method possessed average densities of ~46% before sintering. These samples were sintered at 1373 K in increments of 10 hours up to 50 hours. Based on this temperature and time frame, 1microstructural evolution, 2crystallographic phase analysis, 3transformation behaviors and 4thermal-physical properties were investigated. The additive manufactured Ni-Mn-Ga samples exhibited increases in densities, from ~74% to ~83% due to solid state diffusion mechanisms. X-ray diffraction reveals all of the additive manufactured samples have the 5M martensitic phase at room temperature. Reversible martensitic transformation temperatures were recorded during heating and cooling cycles through differential scanning calorimetry; that indicate austenitic phase transformations occur slightly above ambient temperatures. Additionally, analysis of the heating and cooling cycles prescribe the entropy and Gibb’s energies decrease over the reversible martensitic transformations as sintering time increases. It is envisioned this study will support a more synergistic manufacturing process between binder jetting additive manufacturing and post-heat treatment processes for Ni-Mn-Ga shape memory alloys.

(This is a post-peer-review, pre-copyedit version of an article published in 'Journal of Materials Science'. The final authenticated version is available online at: https://doi.org/10.1007/s10853-020-04352-9. The following terms of use apply: https://www.springer.com/gp/open-access/publication-policies/aam-terms-of-use. )